1. Field of the Invention
The present invention relates to an apparatus for controlling power transmission in a hydrodynamic power transmitting device, such as a slip clutch (which is kept in a prescribed state of slip in order to prevent the transmission of the fluctuation in the output torque of an engine while the clutch is engaged) or a lockup clutch (which is of the direct connection type).
2. Background Art
A clutch of either type is usually provided in parallel with a torque converter attached to the automatic transmission of a vehicle in order to enhance fuel efficiency. In such a conventional transmission system, the output torque on an engine is transmitted through the torque converter while the clutch is disengaged. On the other hand, while the clutch is engaged, the output torque of the engine by-passes the torque converter and is transmitted through the clutch.
In the conventional clutch system, when the clutch is disengaged, applied oil pressure is switched by a valve and delivered to a drain port to quickly eliminate the hydraulic engaging force of the piston of the lockup clutch. For this reason, the output torque of the transmission instantaneously drops to near zero when the lockup clutch is disengaged during the acceleration of the engine. Therefore, the transmission and clutch system has a problem that the response of the vehicle to the acceleration is deteriorated.